An OLED comprises as a rule at least one lower, at least semitransparent electrode, a hole-transporting layer, i.e. a layer comprising charge transport material which is suitable for transporting positive charges, an emitter layer, a layer for transporting negative charges and finally an upper electrode.
In order to manage with as few layers as possible in an OLED, attempts have been made to use, as an emitter layer, an electron-deficient material in which an electron excess can be stabilized. It is thus possible to omit the layer between emitter material and negative electrode.
OLEDs based on semiconductor material having, for example, a skeleton of polyarylenevinylene or poly-para-phenylene (in particular polyfluorene and/or poly-spiro-fluorene), in which, in addition to these constituents, proportions of further chromophores and/or molecules having electronegative centers are also incorporated in the form of polymerized units or introduced in the form of blends are known. The molecules having electronegative centers all have as a rule at least one nitrogen atom by means of the electronegativity of which (in comparison with carbon) an excess electron can be stabilized in the existing π-conjugation.
With the above-described organic semiconductor materials for active layers of an OLED, it is possible at present to cover a large range of the possible CIE (International Commission on Illumination) color coordinates and to achieve relatively high luminances and efficiencies which are relevant for commercial applications. However, the operating life of the polymer OLEDs has to date been too low to become established on the market. This applies in particular to the shorter-wave, i.e. the blue and white emitting light emitting diodes and/or displays, such as the so-called passive matrix displays.
A disadvantage of stabilization by means of a nitrogen atom is, however, that the electron loan pair of the nitrogen has an n-π* transition and an additional electron is therefore raised to a π* energy level and falls into an n-level. It thereby emits radiation which is no longer in the visible range and adds nothing to the luminosity of the OLED.